U.S. Pat. No. 5,919,368, owned by the assignee herein, provides an excellent review regarding the utility of separations by high performance liquid chromatography. As noted therein, the separation process relies on the fact that a number of component solute molecules in a flowing stream of a fluid percolated through a packed bed of particles, known as the stationary phase, can be efficiently separated from one another. The individual components are separated because each component has a different affinity for the stationary phase, leading to a different rate of migration for each component and a different exit time for each component emerging from the column. The separation efficiency is determined by the amount of spreading of the solute band as it traverses the bed or column.
The '368 patent ultimately goes on to describe an improved method of performing liquid chromatography comprising the steps of packing within a tubular container a substantially uniformly distributed multiplicity of rigid, solid, porous particles with chromatographically active surfaces, so as to form a chromatographic column having an interstitial volume between said particles, said particles having average diameters of not less than 30 μm and loading said surfaces with at least one solute that is reactive with said surfaces, by flowing a liquid mixture containing said solute through said column at a velocity sufficient to induce flow of said mixture within at least a substantial portion of said interstitial volume at a reduced velocity greater than about 5000. The aforementioned method: 1. dramatically enhances both the speed and capacity of both analytical and preparative chromatography for both small and large molecules such as biologicals and the like; 2. is operative with mobile phase velocities considerably greater than any previously employed with significantly improved results; 3. makes use of packed particle beds in which the particles are substantially larger than those previously used in the art; and 4. offers a process that is operative at pressures considerably below those taught by the prior art for turbulent flow chromatography. In that regard, attention is also directed to U.S. Pat. Nos. 5,772,874; 5,795,469; 5,968,367; 6,110,362 and 6,149,816, also owned by the assignee herein, which disclose other associated methods and apparatus for use in high performance chromatography applications.
With the above in mind attention is now directed to U.S. Pat. No. 6,066,848 which is said to provide a method and system for analyzing the composition of a plurality of fluid samples by chromatographic techniques. More specifically, the '848 patent states that the invention therein is able to distinguish between each of a plurality of fluid samples simultaneously electrosprayed from parallel liquid chromatography columns towards a miss spectrophotometer such that the mass spectra associated with each fluid sample can be reliable determined. That is, the '848 patent goes on to describe that a plurality of fluid samples are simultaneously separated by parallel liquid chromatography columns and are simultaneously electrosprayed towards an entrance orifice of a mass spectrophotometer. A blocking device having an aperture passing therethrough is positioned so as to block all but one of the fluid samples from passing into the entrance orifice of the mass spectrophotometer at any moment in time.
However, various problems have been identified with the above identified technique, which has in various forms been referred to as a “MUX” (i.e. multiplexed) liquid chromatography/mass spectrometry system. For example, such technology generally makes use of one pump to drive four sample streams. As a consequence, it is difficult to achieve optimum pressure in all streams, the back pressure may vary with column packing, the back pressure may vary with use, there can be poor reproducibility between streams, and there is a standing problem of “drift”. Expanding upon this, there is no back-pressure monitoring, there is no way to link samples to a specific column, and when a single stream shuts down, e.g., in a four-column system, 25% of the data is lost along with 25% of the samples.
In addition, MUX technology, which makes use of a blocking disk, creates a potential for carryover as each spray impinging upon the disk may become cross contaminated. Furthermore, analyte date readout, by virtue of being multiplexed, leads to the feature that each of the electrosprays is analyzed over and over again in sequence. Accordingly, a varying mass spectrum reading will be generated for each of the plurality of fluid sample electrosprays over time. That being the case, MUX analyte data readout will appear as shown in FIGS. 1 and 2 herein, which makes clear that the number of data points is, in all cases, limited by the sampling rate of such a multiplexed system. More specifically, as shown in FIG. 1, data points are a function of sampling rate and are illustrated for sprays 1-4 v. elapsed time. FIG. 2 then illustrates that as a consequence of such multiplexing, the reported peak possibilities are prone to error, as different peaks may be drawn (dotted v. solid line) for the multiplexed data points so collected.
Stated another way, the MUX system can lead to insufficient peak data points for tracing along with a sacrifice in precision and sensitivity when operated at faster speeds. Furthermore, such problems are compounded in the case of multiple analytes.
Accordingly, there remains a need to develop a multi-column chromatography system that overcomes the disadvantages of the prior art and which provides a more reliable and accurate chromatographic separation along with maximum use of a detector's data gathering ability.